In casting using a die casting method, when a hollow portion is formed in a cast product, a sand core, which has a shape corresponding to that of the cast product, is used. For example, in casting using a die casting die 100 shown in FIG. 8, after a sand core 200 is disposed in a cavity 101, die clamping is performed such that a movable die 110 is fixed to a fixing die 120. Molten aluminum is supplied in the cavity 101 at high pressure and high speed, and the molten aluminum is cooled and solidified. In this case, the pressure in the cavity 101 is reduced via a pressure reduction passage 121 in advance, so that a gas therein is discharged and the molten aluminum is supplied. In supplying the molten aluminum and thereafter, remaining gas in the cavity 101 and gas generated from the molten aluminum are discharged via gas drain slits 113 and 123 and a gas vent 122 which are gas drains. Reference numerals 111 and 112 in FIG. 8 are sliding dies which are slidably provided at the movable die 110.
The sand core 200 used in this high pressure casting is equipped with a core body formed such that particles of silica sand (including SiO2 as a main component) or the like are connected by an organic binder of phenolic resin or the like. A surface of the sand core 200 is covered with a coating layer. The coating layer is formed in order to prevent infiltration of the molten metal into the core body and to allow easy separation of the cast product and the sand core. In particular, in the casting using the above die casting method, the molten metal is supplied at high pressure, so that the coating layer is important for preventing infiltration of the molten metal into the core body. This coating layer includes the organic binder in order to connect main components thereof (for example, fire-resistant powdered materials or mica) to each other and to connect the coating layer and the core body.
The temperature of the above organic binder in the core body and the coating layer (in particular, in the coating layer) increases in the casting and the organic binder combusts, so that the organic binder is decomposed into low molecular gases (carbon monoxide, carbon dioxide, water, and the like), and the low molecular gases are discharged via the gas drain slits 113 and 123 and the gas vent 122.
However, tar, soot, and the like may be generated by incomplete combustion of the organic binder. In particular, the pressure in the cavity 101 is reduced in the above manner, air and oxygen may not be supplied from the outside to the cavity 101, and the sand core 200 is integrally cast with aluminum which is a material of the molten metal, so that oxygen is insufficient in the surroundings of the sand core 200. Due to this, incomplete combustion of the organic binder may occur, and tar, soot, and the like may be generated, so that they may adhere to the gas drain slits 113 and 123 and the gas vent 122 which are gas drains. Thus, clogging may occur therein, so that gas discharge may be inhibited, and defects may be generated in cast product due to gas entrapment.
In order to prevent clogging in the gas drains, various techniques have been proposed. For example, as disclosed in Japanese Unexamined Patent Application Publication No. 2007-105738, a technique has been proposed in which a duct plug, which is composed of an oxygen nonstoichiometric ceramic, is disposed in a gas drain. In the technique in Japanese Unexamined Patent Application Publication No. 2007-105738, a tar, which is in a gas circulating in the gas drain, is reacted with oxygen discharged from the ceramic, and the tar thereby burns. Thus, the tar is decomposed into low molecular gases (carbon monoxide, carbon dioxide, water, and the like), and the low molecular gases are discharged via the gas drain.
As disclosed in Japanese Unexamined Patent Application Publication No. 2007-136475, a technique has been proposed in which a molten metal infiltration prevention pin is inserted in a gas drain such that a predetermined interval is formed between the molten metal infiltration prevention pin and an inner peripheral surface of the gas drain, and a blade is provided on a peripheral surface of the molten metal infiltration prevention pin. In the technique in Japanese Unexamined Patent Application Publication No. 2007-136475, rotation of the molten metal infiltration prevention pin is driven, and tar adhering to the gas drain is removed by the blade provided on the peripheral surface of the molten metal infiltration prevention pin.
However, in the technique in Japanese Unexamined Patent Application Publication No. 2007-105738, it is necessary to furthermore provide the duct plug to the die, and it is also necessary to use a member such as a heating device to burn the tar in the gas drain. In the technique in Japanese Unexamined Patent Application Publication No. 2007-136475, the blade is furthermore provided on the peripheral surface of the molten metal infiltration prevention pin inserted in the gas drain. In the techniques in Japanese Unexamined Patent Application Publications Nos. 2007-105738 and 2007-136475, it is necessary to furthermore provide the tar removing member in the gas drain or the surroundings thereof. Due to this, the structure may be complicated.